Die-casting of iron in chill-moulds



U tat s, P O

1 2,906,653 DIE-CASTING OF IRON IN CHILL-MOULDS Lucien Peras,Billanconrt, France, assignor to Regie Natiouale des Usines Renault,Billancourt, France No Drawing. Application December 27, 1955 Serial No.555,285 Claims priority, application France February 1, 1955 13 Claims.(Cl. 148-213) In the patent application Ser. No. 336,188 of February 10,1953, a process was described for manufacturing parts in ferrous metalshaving high mechanical properties and very accurate dimensions,according to which the white iron castings are made by die-casting inchillmoulds, said castings then being subjected to a heat treatment inthree phases: quenching, nucleation, graphitization.

Thus, as stated in said application, the combination of the processes ofchill-casting, die-casting, and graphitization with pre-treatment forthe nuclear formation, leads industrially to new products of highquality, high output of mass-produced parts, precision and good surfacequalities on the castings, high strength of the cast iron and regularityof production.

In fact, the die-casting of iron in chill-molds is greatly facilitated,compared with steel, by the great stability of the metal bath in thestorage furnace, by the lowering of the casting temperature, and by theconsiderable reduction in the effects of hot erosion on the chill-mold.

Thus, white iron can be cast without micro-shrinkagecracks orblow-holes, thanks to the pressure exerted during the solidification.Cracks are avoided by stripping immediately after the solidification isfinished.

Due to the chill-casting, the iron can have a relatively high siliconcontent which lowers the tapping temperature while preserving a whitestructure in the rough-cast pieces. Thus the normal content in malleablecast irons can be considerably exceeded, reaching values of between 1.5and 2%. Moreover, higher carbon contents of 2.6 to 3% can be allowed forthe same reasons.

In order to obtain the optimum effect, it is preferable, according tothe invention, to use irons containing copper though without that beingan absolute necessity. Copper acts as a constituent to improve thecasting qualities, as an agent for the nuclear formation of thegraphite, and as an additive with a view to suitability for thehardening and tempering heat-treatment.

In order for the process to have its full effect, it is necessary,according to the invention, to carry out the triple treatment ofhardening, nuclear formation and graphitization. The hardening treatmentcan be carried out, starting from the casting heat, by stripping fairlyhot, above 810 and quenching directly in a salt-bath, for example at 180for 1 minute, then cooling in still air, which makes it possible toobtain chilled castings without the risk of cracks or shrinkage cracks.If the casting comprises narrow parts which have been overcooled, itcan, after being stripped, be immersed in a stabilizing bath at 810 oneminute, after being salthardened at 180. The casting is then subjectedto nuclear formation treatment, for example 48 hours at 450", then, withor without intermediate cooling, the casting is subjected tographitization of the primary cementite; in order for the graphitizationto be absolutely complete. it is necessary to maintain it at 875 for 2to 6 hours, but this time can be reduced to between 40 minutes and twohours by annealing at 900.

The following table gives the number N per mm. of fine spherules ofgraphite having an average diameter of 2 to 6 microns obtained by thisprocess. In these tests, the chill-casting was 14 mm. thick; strippingwas done at more than 820 and the piece was hardened directly ice insalt at The annealing comprised a first cycle of 48 hours at 450, and asecond: 3 hours at 875.

In order for the process to attain its full effectiveness, it ispreferable to allow the piece to cool on leaving the matrix and toreheat it for austenization at 810 for 30 minutes, for example, andhardening in stages, at 180 for 1 minute for example. The castings arethen subjected to nuclear formation treatment, for example 48 hours at450, cooled, then reheated to 875 for example, just long enough for thegraphitization of the primary cementite, then cooled in still air,failing which, if the time taken is too long, the graphite undergoes acoalescence with a reduction in the number of spherules and lowering ofthe mechanical properties. This is shown by the following table whichgives the number of spherules of graphite per mm. having a diameter ofless than 2 microns:

In each case, the quantity of cementite is nil, so that the annealingfor 1 hour at 875 is suflicient. It will be seen that here the bestresult is obtained with the cast iron containing 1.2% copper, which has40,000 spherules of graphite per mm.

These chill-cast irons, reheated 810/ 30 minutes, hardenecl in salt at180 for 1 minute, cooled in still air, reheated to 450 for 48 hours,cooled in still air then reheated at 875 for 1 hour, and cooled in stillair, are lamellar pearlitic and have the following characteristics intest-pieces 4 mm. in diameter, machined from pieces 20 mm. thick:

After this treatment, a piece of casting 2162 was reheated to 835,hardened in oil, tempered at 700; it then showed: 12:80, 12:82, A=2.5%in a machined test-piece 4 mm. in diameter.

it is also possible, after this last tempering at 700, to stop this byoil-hardening and temper it for 2 hours at 500 to induce the structuralhardening of the copper.

Finally, it is possible to modify the compositions and properties ofthese cast irons by alloys such as: Ni, Mo, Ti, Al, Zr and the like. Themachinability, after graphitization, with or without hardening andtempering, is particularly easy, due to the graphite.

The copper content of the casting is suitably 0.4 to 3% and ispreferably 1 to 2%, as stated in said copending application.

Further research has shown that when the castings obtained according tothe aforesaid patent application by die-casting in chill-moulds aresubjected to a heat treatment in a simple cycle, of the usual type inthe standard manufacture of malleable cast iron, diffuse distribution ofgraphite is effected and mechanical properties are obtained, which,without reaching the exceptional values permittedby the three-phasetreatment, are greatly superior to those obtained in the normalmanufacture of malleable cast iron, and suflicient for certainapplications.

The following table gives, by way of example, a comparison of the numberof spherules of graphite per mm. and of the mechanical properties ofcastings having the same composition as follows: C=2.50%, Si=1.50%,Mn=0.50%, O1=0.40%, some cast in sand as is the usual practice in themanufacture of malleable cast iron and treated in known manner byheating for 18 hours at 940 C followed by cooling in the furnace to 740C,, which temperature is maintained for 24 hours, followed finally bycooling in air; the others die-cast in chill-moulds and treated eitherby the usual treatment which has just been described or by thethree-phase treatment: quenching, nucleation, graphitization.

In this table, E represents the elastic limit, R the breaking strength,and A the elongation, according to the French standards for steels.

The subject of the present invention is therefore a process whichconsists, as a modification of that described in the aforesaid patentapplication, in proceeding successively with die-casting in chill-mouldsto obtain white iron, and with a graphitization treatment in a singleheat cycle comprising annealing to a temperature above 850 C., for aperiod comprised between 30 minutes and 6 hours, which may be followedby a holding at a temperature not very different from A for a periodcomprised .between and 8 hours, finally followed by cooling in air. Thislast holding at a temperature in the vicinity of A is useless if a toughproduct with a pearlitic structure is desired. It is used to obtain amalleable ferritic structure, the period of holding being selectedbetween 0 and 8 hours depending on the desired proportion of pearliteand ferrite constituents, in other words depending on the balance whichhas to be obtained between toughness .andmalleability, for a givenapplication.

For concerns which have furnaces for the conventional treatment ofmalleable cast iron, at their disposal, the new process has theadvantage of enabling castings to be obtained having precise dimensionsand high mechanical properties, without investing in neaw heat-treatmentapparatus.

Another advantage results from the fact that these mechanicalproperties, which are superior to those of ordinary malleable cast iron,are obtained, in the treatment according to the invention, by a shorterannealing time. For example, for the complete graphitization of Ltheprimary cementite, the minimum period at 950 C. would be 18 hours forordinary malleable cast iron, whereas a period of two hours at the sametemperature is sufficient for the new process.

Cooling from the annealing temperature may be reg- 11lated in .such amanner that the structure of the matrix is wholly or partiallypearlitic. The hardness and toughness of the metal can thus be regulatedto any desired value.

1. A method of obtaining castings of pearlitic structure containingdiifused fine particles of graphite which comprises die casting in achill mold an iron suitable for the formation of white iron, strippingthe casting from the mold while hot, and heat-treating the casting in asingle cycle comprising the steps of annealing to a temperature above850 C. for A2 to 6'hours, and cooling in air.

2. A method of obtaining castings of ferritic structure containingdiffused fine particles of graphite which comprises die casting in achill mold an iron suitable for the formation of white iron, strippingthe casting from the mold while hot, and heat-treating the casting in asingle cycle comprising the steps of annealing to a temperature above850 C. for /2 to 6 hours, and cooling in air.

3. A method of obtaining iron castings of pearlitic and ferriticstructure containing diffused fine particles of graphite which comprisesdie casting in a chill mold an iron suitable for the formation of whiteiron, stripping the casting from the mold while hot, and heat-treatingthe castingin a single cycle comprising the steps of annealing to atemperature above 850 C. for /2 to 6 hours, holding the casting at atemperature approximating the A transformation point for a period up to8 hours, and cooling in air, the formation of the ferritic structurebeing greater the longer the holding of said casting at said last-namedtemperature.

4. A process as defined in claim 1, treated contains 0.4% to 3% copper.

5. A process as defined in claim 1, treated contains 1% to 2% copper.

6. A process as defined in claim 2, treated contains 0.4% to 3% copper.

7. A process as defined in claim 2, treated contains 1% to 2% copper.

8. A process as defined in claim 3, treated contains 0.4% to 3% copper.

9. A process as defined in claim 3, treated contains 1% to 2% copper.

10. A method of obtaining castings containing diffused fine particles ofgraphite which comprises die casting in a chill mold an iron suitablefor the formation of white iron, stripping the casting from the moldwhile hot, and heat-treating the casting in a single cycle comprisingthe steps of annealing to a temperature above 850 C. for A2 to 6 hours,and cooling in air.

11. A method of obtaining iron castings containing diffused fineparticles of graphite which comprises die casting in a chill mold aniron suitable for the formation of white iron, stripping the castingfrom the mold while hot, and heat-treating the casting in a single cyclecomprising the steps of annealing to a temperature above 850 C. for /2to 6 hours, holding the casting at a temperature approximating the A,transformation point for a period up to 8hours, and cooling in air.

12. A process as defined in claim 10, wherein the iron treated contains0.4% to 3% copper.

13. A process as defined in claim 10, wherein the iron treated contains1% to 2% copper.

wherein the iron wherein the iron wherein the iron wherein the ironwherein the iron wherein the iron References Cited in the file of thispatent UNITED STATES PATENTS 1,498,128 Sowers June 17, 1924 1,747,728Morris et al. Feb. 18, 1930 1,815,361 Morris et al. July 21, 19312,185,894 Hultgren Jan. 2, 1940 2,331,886 Boegehold Oct. 19, 1943FOREIGN PATENTS 999,242 France Oct. 3, 1951 OTHER REFERENCES Die CastingPractice, page 4. Edited by Stern. 'Published in 1930 by McGraw-HillBook Co., NY.

Die Casting, Machines, Dies, Alloys, pages 289 to 292. Copyright 1936.Edited by Herb. Published by the Industrial Press, 148 La FayetteStreet, New York, NY.

Metals Handbook, 1948 edition, page 5. Published by the American Societyfor Metals, Cleveland, Ohio.

1. A METHOD OF OBTAINING CASTINGS OF PEARLITIC STRUCTURE CONTAININGDIFFUSED FINE PARTICLES OF GRAPHITE WHICH COMPRISES DIE CASTING IN ACHILL MOLD AN IRON SUITABLE FOR THE FORMATION OF WHITE IRON, STRIPPINGTHE CASTING FROM THE MOLD WHILE HOT, AND HEAT-TREATING THE CASTING IN ASINGLE CYCLE COMPRISING THE STEPS OF ANNEALING TO A TEMPERATURE ABOVE850* C. FOR 1/2 TO 6 HOURS, AND COOLING IN AIR.